The present invention relates to light-emitting semiconductor elements and, more specifically, to light-emitting semiconductor elements such as light-emitting diodes effectively usable for plastic fiber optic communications according to IEEE1394 and USB-2 standards.
Among group III-V compound semiconductor materials except for nitrides, industry attention has been focused on AlGaInP compound materials as of light-emitting semiconductor elements which possess the largest direct-transition band gap and a large emission output with wavelengths of 0.5 to 0.6 xcexcm. Especially, p-n junction type light-emitting diodes (LED) each having a light-emitting portion (laminated structure including an active layer) formed by lattice matching crystal growth on a substrate GaAs are capable of emitting light having wavelengths corresponding to that of red and green at a higher response speed and greater output in comparison with conventional LEDs made of GaP and AlGaAs materials. Hence, they can be effectively used as light sources for plastic fiber optic transmission and communications systems.
FIG. 1 is a sectional view of a first conventional light-emitting diode having a light-emitting portion of AlGaInP material.
The structure of the first conventional LED of AlGaInP is described bellow.
As shown in FIG. 1, the first conventional LED comprises an n-type GaAs substrate with subsequently formed thereon layers, i.e., an n-type GaAs buffer layer 2, an light-reflecting layer 3 (a DBR layer of carrier concentration of 5xc3x971017 cmxe2x88x923) composed of an n-type (AlxGa1xe2x88x92x) As (x=0.45) and an n-type AlAs, a lower clad layer 4 of n-type (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) (e.g., x=1.0, width 1.0 xcexcm and carrier concentration 5xc3x971017 cmxe2x88x923), an active layer 5 of p-type (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) (e.g.,x=0.0, width 0.1 xcexcm and carrier concentration 5xc3x971017 cmxe2x88x923), an upper clad layer 6 of p-type (AlxGa1xe2x88x92x)0.51In0.49P)(0xe2x89xa6xxe2x89xa61) (e.g., x=1.0, width 1.0 xcexcm and carrier concentration 5xc3x971017 cmxe2x88x923), an intermediate layer 7 of p-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (x=0.2, y=0.4, thickness 0.15 xcexcm and carrier concentration 1xc3x97108 cmxe2x88x923), a current spreading layer 9 of p-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (x=0.05, y=0.95, thickness 1.5 xcexcm and carrier concentration 5xc3x971018 cmxe2x88x923), and a current blocking layer 8 of n-type GaP (thickness 0.3 xcexcm and carrier concentration 1xc3x971018 cmxe2x88x923).
Then, the n-type GaP current blocking layer 8 is selectively etched by photolithography to form a current path of 50 to 150 xcexcmxcfx86 in diameter therein, and the current spreading layer 9 of p-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP (x=0.05, y=0.05, thickness 7 xcexcm and carrier concentration 5xc3x971018 cmxe2x88x923) is grown again to complete the element structure.
On the p-type current spreading layer 9, a film of, e.g., Auxe2x80x94Be is deposited and then circularly patterned to form a p-type electrode 10 and a light-emission area.
On the other hand, an n-type electrode 11 of, e.g., Auxe2x80x94Ge film is formed by deposition on the bottom surface of the GaAs substrate 1.
Thus, the first conventional light-emitting diode is completed.
The mixed crystal ratio xe2x80x9c1xe2x88x92yxe2x80x9d of indium (In) in the lower clad layer 4 of n-type AlyIn1xe2x88x92yP, the active layer 5 of p-type GayIn1xe2x88x92yP and the upper clad layer 6 of p-type AlyIn1xe2x88x92yP is set to 0.5 so as to match the crystal lattice of the GaAs substrate 1.
The p-type AlGaInP current spreading layer 9 in a (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor is transparent to a light-emission range of wavelengths of 550 nm to 650 nm and a low resistance. To easily make an ohmic contact with the p-side electrode, the mixed crystal ratio of Al and the mixed crystal ratio of In are set to lower values, usually x=0.05 and y=0.95 respectively.
Generally, in LEDs of AlGaInP, Si is used as an n-type dopant and Zn is used as a p-type dopant.
The (AlxGa1xe2x88x92x)yIn1xe2x88x92y active layer 5 is usually of the p-type. Since the element is forced to emit light of 650 nm to 670 nm that is hardly absorbed in a plastic optical fiber, the mixed crystal ratio of Al in the active layer is selected within the range of 0.03 to 0.00. Since the thinner active layer may confine injected carriers to a smaller area thereof to increase the density of current therein and the higher density of carriers may shorten the lifetime of carrier recombination, the thickness of the active layer is set to 0.1 xcexcm or less and the density of carriers therein is set to 7xc3x971017 cmxe2x88x923 or more. However, LEDs of AlGaInP involve such a problem that decreasing of the thickness of the active layer and/or increasing of doping concentration is accompanied in practice by decreasing of initial radiant output light power and intensive variation (increase or decrease) of output light power during the operation.
Recently, there has been proposed a LED having a quantum well structure of an active layer 5 for increasing radiant output light power and having an increased current density for improving the response characteristic.
FIG. 2 is a sectional view of a second conventional light-emitting diode.
Referring to FIG. 2, the quantum well structure of the second conventional light-emitting diode is described below.
The LED shown in FIG. 2 differs from the first LED of FIG. 1 by merely its active layer portion that, in replace of p-type (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) active layer 5, comprises an undoped (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) lower barrier layer 12 (e.g., x=0.3, width 500 xc3x85 and carrier concentration 1xc3x971016 cmxe2x88x923), an undoped (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) upper barrier layer 14 (e.g., x=0.3, width 500 xc3x85 and carrier concentration 1xc3x971016 cmxe2x88x923) and 4-10 undoped (AlxGa1xe2x88x92x)0.51In0.49P (0xe2x89xa6xxe2x89xa61) quantum-well layers 13 (e.g., x=0.0, width 80 xc3x85 and carrier concentration 1xc3x971016 cmxe2x88x923) sandwiched between the two barrier layers.
The quantum well structure of the active layer is described below with reference to FIG. 3 depicting energy bands therein.
4 quantum-well layers 13 each having 80 xc3x85 in thickness is sandwiched between the lower barrier layer 12 and the upper barrier layer 14.
The LED having the quantum well type active layer may have a small overflow of injection carriers and attain a higher light output than that of the conventional LED of FIG. 1. However, this LED cannot attain high-speed response characteristic because of a large number of wells, decreasing the density of injected carriers and elongating the lifetime of carrier recombination. In other words, the multiple quantum-well structure of the LED is advantageous to attain a high light output but disadvantageous to improve the response characteristic. In contrast to the first conventional LED, the second conventional LED cannot possess a high-speed response characteristic.
As described above, the LED having a usual bulk type active layer may have high-speed response characteristic by reducing the thickness of the active layer and increasing the carrier concentration but cannot attain a sufficient initial radiant output light power and a stable light emission during the operation.
On the contrary, the LED having the multi-quantum-well structure may attain a sufficient initial light output and a stable light emission during energizing period but may not attain a high-speed response characteristic.
An object of the present invention is to provide a light-emitting diode that has an excellent high-speed response characteristic as well as a large output light power and a very small variation of light emission during its operation.
Another object of the present invention is to provide a light-emitting semiconductor element wherein a half width of a spectrum of light emitted from an active layer is not less than 25 nm and a rise time and a fall time of a rectangle pulse current are each of no more than 4 ns.
Another object of the present invention is to provide a light-emitting semiconductor element, wherein the active layer comprises a single quantum well layer and two barrier layers sandwiching the quantum well layer therebetween and said active layer is doped with p-type or n-type impurity.
Another object of the present invention is to provide a light-emitting semiconductor element, wherein the carrier concentration of the dopant impurity is within a range of 1xc3x971017 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923.
Another object of the present invention is to provide a light-emitting semiconductor element, wherein the active layer is doped with the impurity by a method for diffusing the dopant thereto.
Another object of the present invention is to provide a light-emitting semiconductor element, wherein material of the p-type impurity is any one of Zn, Mg, Be and C and material of the n-type impurity is any one of Si, Se and Te.
Another object of the present invention is to provide a light-emitting semiconductor element, wherein a current diffusing layer or a cap layer is formed by regrowth.
Another object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein a half width of light emitted from an active layer of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material is not less than 25 nm and a rise time and a fall time of a rectangle pulse current are each of no more than 4 ns.
Another object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein a mixed crystal ratio 1-y of In in the quantum well layer of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP is not less than 0.5 for applying a distortion by a compressive strain to the quantum well layer to produce a longer light wave therefrom.
Another object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein a mixed crystal ratio x of Al in the barrier layers of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP disposed on both sides of the quantum well layer of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP is set to a value within a range of 0.4 to 1.0.
Another object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein thickness of each of the barrier layers of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP disposed on both sides of the quantum well layer of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP is set to a value within a range of 50 xc3x85 to 300 xc3x85.
A further object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein a lattice-unmatched active layer is grown on a substrate tilted at an angle of not less than 5xc2x0 and not more than 15xc2x0 for a direction (011).
A Still further object of the present invention is to provide a surface-emission type light-emitting semiconductor element, wherein there is a single quantum-well active layer grown on a light-reflecting layer.